A term mostly used by gay men to describe something good (example 1) or the way someone is dressed (example 2 and 2a). The tone of voice also matters (especially in example 2a)
Person 1: "How was that party girl?"
Person 2: "Girl, that party was FIERCE as hell!"
"Work! That outfit is FIERCE girl! You look good!"
"Hmph! No...that outfit...is FIERCE, you need to take that off right now! It is so ova